I. Field of the Invention
The present invention relates generally to chucks for holding a workpiece during a machining operation upon the workpiece and, more particularly, to such a chuck having a jaw which is displaced by a rotatably driven member.
II. Description of the Prior Art
Accurate machining of workpieces often requires that the workpiece be clamped in a stationary position so that a cutting tool, abuts against and machines the workpiece. The rigid clamping of the workpiece becomes especially important, although more difficult, when the workpiece is irregularly shaped and/or includes a plurality of surfaces to be machined such as a crankshaft on which the main bearings as well as the crank pins must be machined.
Since the crankshaft main bearings are aligned on the major axis of the crankshaft, the ends of the crankshaft are conventionally held by coaxially aligned spindles. The crankshaft is then rotated between the spindles while a cutting tool machines the main bearing surfaces. However, the crank pins of the crankshaft lie on an axis parallel to but spaced apart from the major axis of the crankshaft. Thus, the crankshaft must be rotated about the axis of the crank pins in order to machine the crank pins.
A previously known chuck device for holding crankshafts during a machining operation comprises two clamping jaws which engage the main bearing or the crank pin of the crankshaft. The jaws are attached to one end of a pivoting lever while the other end of the lever contacts a cam surface of a linearly movable member which is driven by a piston and cylinder arrangement. An example of such a chuck is described in U.S. Pat. Nos. 4,171,654 and 4,099,431 to Kreucher.
These previously known chuck devices, however, are disadvantageous in that they are complicated by the fact that the motivating force for closing and opening is a linear actuating device whose motion must be changed to a rotary motion by means of cams. Such previously know devices are also bulky and require frequent maintenance.
Another previously known chuck is disclosed in U.S. Pat. No. 4,044,638 to Heffron et al in which three radially spaced chuck jaws are shown and in which each jaw is secured to one end of a linearly movable arm which controls the actuation of the jaws. This type of previously known chuck suffers substantially the same disadvantages as the previously discussed chuck. Moreover, the use of three moving jaws requires accurate alignment of each of the jaws and thus makes the device extremely complicated and expensive.